Tin-silver based soldering alloy utility

ABSTRACT

A tin-silver-based soldering alloy, comprises 3.0 to 4.0% by weight of Ag, 6.05 to 10.0% by weight of In, 2.0 to 5.95% by weight of Bi, and the balance being Sn, wherein In+Bi≦12.0% by weight.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a tin-silver-based soldering alloy, and more particularly, to a tin-silver-based soldering alloy having a melting point of 210° C. or lower, i.e., a low melting point, and excellent in mechanical properties such as tensile strength and elongation.

[0003] 2. Description of the Related Art

[0004] Conventionally, an alloy having a eutectic composition near Pb—Sn has been known as a typical soldering alloy. Further, an alloy made of Zn—Cd having a higher strength than that of a Pb—Sn eutectic soldering alloy or the like has also been known. However, the former soldering alloy has a problem of toxicity of lead and the latter soldering alloy has a problem of an adverse effect of cadmium evaporation to an operator. Therefore, neither has solved these recent environmental problems.

[0005] Under such circumstances, there have been proposed various types of tin-silver-based soldering alloy not containing harmful Pb, Cd, or the like. For example, there is known “Alloy H” comprising 2.0% by weight of Ag, 0.5% by weight of Cu, 7.5% by weight of Bi, and the balance being Sn. The “Alloy H” has a melting point which is higher than that of the Pb—Sn eutectic soldering alloy and lower than that of the conventional tin-silver-based soldering alloy not containing Pb, Cd, or the like, i.e., 212° C., and thus excellent in fusing properties. However, the elongation, i.e., mechanical property of “Alloy H” is low. Therefore, the “Alloy H” cannot absorb the thermal expansion difference between a substrate and a part when the soldered portion is subjected to a temperature cycles. Consequently, the soldered portion might be broken.

[0006] From such a viewpoint, there is proposed a tin-silver-based soldering alloy containing Bi and In in respective given amounts as a soldering alloy excellent in mechanical properties such as tensile strength and elongation (Japanese Patent Laid-Open Publication No.71488/98). However, this tin-silver-based soldering alloy is excellent in mechanical properties, but has a high melting point. Therefore, a soldering alloy usable at lower temperatures is desired.

SUMMARY OF THE INVENTION

[0007] Accordingly, an object of the present invention to provide a soldering alloy having 5 a low melting point without degrading the mechanical properties.

[0008] As a result of extensive studies, the present inventors found that the above object can be achieved by setting the contents of bismuth and indium so as to satisfy a given relationship in a tin-silver-based soldering alloy containing bismuth and indium in respective given amounts.

[0009] The present invention has been achieved based on the above findings, and provides a tin-silver-based soldering alloy comprising 3.0 to 4.0% by weight of Ag, 6.05 to 10.0% by weight of In, 2.0 to 5.95% by weight of Bi, and the balance being Sn, and wherein In+Bi≦12.0% by weight.

[0010] The tin-silver-based soldering alloy of the present invention contains no harmful Pb, Cd, and the like, will not degrade the mechanical properties, and has a low melting point. Therefore, the soldering temperature can be lowered. Further, since the tin-silver-based soldering alloy of the present invention exhibits no low-temperature eucteric peak, the thermal stress characteristic can be improved. In addition, it is also unnecessary that the tin-silver-based soldering alloy of the present invention should further contain another element.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will now be described hereunder in detail.

[0012] The Ag content of the tin-silver-based soldering alloy of the present invention is 3.0 to 4.0% by weight, and optimally 3.5% by weight, but it falls within the above range from the viewpoint of the manufacturing yield during manufacturing of the soldering alloy. The In content is 6.05 to 10.0% by weight, preferably 6.2 to 10.0% by weight, and more preferably 7.0 to 10.0% by weight. If the In content is less than 6.05% by weight, the melting point becomes higher. If it exceeds 10.0% by weight, a low-temperature eutectic peak appears, and the thermal stress characteristic is degraded. Further, the Bi content is 2.0 to 5.95% by weight. If the Bi content is less than 2.0% by weight, the melting point becomes higher. If it exceeds 5.95% by weight, the elongation is reduced.

[0013] In the tin-silver-based soldering alloy of the present invention, it is required for In and Bi to have the following relationship. That is, In+Bi≦12.0% by weight. If In+Bi>12% by weight, the degradation in elongation becomes significant. Incidentally, in the case where Bi and In are added individually as third elements, the low-temperature eutectic peak is observed. However, the low-temperature eutectic peak disappears by adding both the elements in combination so as to satisfy the above relationship and the thermal stress characteristic is improved. In addition, it is not necessary that the present invention should further contain another element, e.g. Sb. If the present invention contains such an element, mechanical properties such as elongation and tensile strength becomes inferior.

[0014] The present invention will be illustrated in great detail by way of Examples.

EXAMPLES 1-8 AND COMPARATIVE EXAMPLES 1-4

[0015] Sn, Ag, Bi, and In were weighed for a total weight of 10 Kg so as to have a composition shown in Tables 1 and 2, and the mixture was molten in an electric furnace in the atmosphere using a graphite crucible. The melting temperature was set at 300° C. The molten mixture was allowed to cool, and the melting point thereof was measured by means of a thermocouple-type thermometer. The results are shown in Tables 1 and 2. It is indicated that the data including a numerical value written on the right of a slash refers to the case where a low-temperature eutectic occurred, and the numerical value thereof denotes the peak temperature of the low-temperature eutectic.

[0016] Further, after each metal was completely molten, the mixture was stirred sufficiently to avoid gravity segregation, and cast in a mold having inner dimensions of 150×60 mm and a height of 150 mm, with a mold thickness of 10 mm. From the lower part of the casting obtained, a JIS No. 4 test piece was taken by machining and evaluated for the elongation of the initial stage of casting and the elongation after the heat treatment at 150° C. for 300 hours, in accordance with a test method based on JIS Z2241. The results are shown in Tables 1 and 2. Numerical values in connection with the composition of Tables 1 and 2 are on the basis of % by weight. TABLE 1 Elongation (%) After heat Indication of Melting Initial treatment at Elongation Composition Point stage of 150° C. for Degradation (wt. %) (° C.) casting = *1 300 hours = *2 *2/*1 Remarks Ex. 1 Sn-3.5Ag-7.5In-2.3Bi 205 23 21 0.91 In ≧ 6.05 2 Sn-3.5Ag-7.0In-2.5Bi 205 23 21  0.91) 3 Sn-3.5Ag-6.5In-2.7Bi 205 22 21 0.95 4 Sn-3.5Ag-6.2In-3.0Bi 205 22 21 0.95 5 Sn-3.5Ag-6.05In-3.2Bi 205 21 20 0.95 Com. 1 Sn-3.5Ag-6.0In-3.9Bi 205 19 16 0.84 In < 6.05 Ex. 2 Sn-3.5Ag-5.5In-4.1Bi 205 18 14 0.78 3 Sn-3.5Ag-4.5In-5.4Bi 205/ 13  5 0.38 101

[0017] TABLE 2 Elongation (%) After heat Indication of Melting Initial treatment at Elongation Composition Point stage of 150° C. for Degradation (wt. %) (° C.) casting = *1 300 hours = *2 *2/*1 Remarks Ex. 6 Sn-3.5Ag-6.5In-3.0Bi 205 23 21 0.95 In + Bi ≦ 12 7 Sn-3.5Ag-6.5In-5.0Bi 201 12 11 0.92 8 Sn-3.5Ag-8.0In-3.0Bi 201 18 18 1.00 Com. 4 Sn-3.5Ag-7.0In-6.0Bi 199  7  5 0.71 In + Bi > 12 Ex.

[0018] As apparent from the results shown in Table 1, the soldering alloys of Examples 1-5 have such a low melting point as 205° C., large elongation with small elongation degradation. In contrast, the soldering alloy of Comparative Examples 1-3 having indium in an amount of less than 6.05% by weight have the same level of melting point but small elongation with elongation degradation being extremely large, compared with the soldering alloy of Examples 1-5. In addition, low-temperature eutectic occurs in the soldering alloy of Comparative Example 3.

[0019] As apparent from the results shown in Table 2, the soldering alloy of Examples 6-8 have a low melting point, and their elongation falls within the desirable range with the elongation degradation being small. In contrast, the soldering alloy of Comparative Example 4 wherein In+Bi>12.0, has a low melting point but the elongation is small with the elongation degradation being extremely large, compared with the soldering alloy of Examples 6-8.

EXAMPLE 9, COMPARATIVE EXAMPLE 5 AND REFERENTIAL EXAMPLE 1

[0020] A tin-silver-based soldering alloy having the composition shown in Table 3 was obtained in the same manner as that of Examples 1-8 and Comparative Examples 1-4.

[0021] Wettability of the tin-silver-based soldering alloy thus obtained was measures by means of meniscograph method and evaluated with 0 cross time. The results are shown in Table 3. The term “0 cross time “represents wetting time, wherein shorter the time, easier to wet and more serviceable as solder. The test condition is follows: Numerical values in connection with the composition of Table 3 are on the basis of % by weight.

[0022] Test Condition

[0023] 0.9 mm φ Cu line RA flux

[0024] Test temperature; 240° C.

[0025] Immersion time: 5 sec.

[0026] Immersion depth; 2 mm

[0027] Immersion rate; 2 mm/sec. TABLE 3 Composition (wt. %) 0 cross time (sec.) Example 9 Sn-3.5Ag-6.5In-3.0Bi 1.22 Comparative Sn-3.5Ag-5.5In-3.0Bi 1.50 Example 5 Referential Alloy H 1.35 Example 1

[0028] As apparent from the results shown in Table 3, the soldering alloy of Example 9 is superior to Alloy H (Referential Example 1) which is a conventional soldering alloy, in wettability. In contrast, the soldering alloy of Comparative Example 5 having Indium in an amount of less than 6.05% by weight is inferior to Referential Example 1 in soldering wettability. 

What is claimed is:
 1. A tin-silver-based soldering alloy, comprising 3.0 to 4.0% by weight of Ag, 6.05 to 10.0% by weight of In, 2.0 to 5.95% by weight of Bi, and the balance being Sn, wherein In+Bi≦12.0% by weight.
 2. The tin-silver-based soldering alloy according to claim 1 , wherein the In content is 6.2 to 10.0% by weight. 